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Modems. Chapter 17. Modems are little devices to use the telephone to talk to other computers. Modem is an abbreviation for Modulator/ Demodulator Modems can be internal or external. Basic Knowledge. Modem Signals.

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Chapter 17

basic knowledge

Modems are little devices to use the telephone to talk to other computers.

  • Modem is an abbreviation for Modulator/ Demodulator
  • Modems can be internal or external.

Basic Knowledge

modem signals
Modem Signals
  • Modems take analog signals and convert them into digital signals and vice versa.
  • The data that goes into the modem is called serial communication and is a series of 0’s and 1’s. This data is converted into bytes via the UART(Universal Asynchronous Receiver/Transmitter) so CPU’s can understand information
  • There are 2 ways of “chopping up” the incoming signals
    • Asynchronous
    • Synchronous
asynchronous organization
Asynchronous Organization
  • The main way to serial ports communicate
  • The data is “chopped up” into 7 or 8 bit packets. 8-bit is most common
  • Each Packet starts with a start bit that tells the receiving modem that it is the beginning of a piece of information and it always is zero and ends with a stop bit that tells the receiver the packet is over.
  • Asynchronous communication packets have an optional parity bit used for error detection. Parity bits are rare
  • Asynchronous packet settings are described in a particular order: data bits, parity type, then stop bits.
    • Example: If a system uses 8 data bits, no parity, and 1 stop bit, you’d say 8-N-1.
synchronous organization
Synchronous Organization
  • Mainly for uploading and downloading programs to and from your computer.
  • Protocols are sets of definitions used by communication programs so that both computers are speaking the same language. There are many protocols that basically work the same way.
  • When a modem is going to send data with synchronous communication it sends a series of standardized bytes called sync bytes, which tell the receiving port that it is about to get information.
  • After sync bytes the sending modem adds a start-of-text(STX) marker then sends the data.
  • After data transmission the packet ends with an end-of-text(ETX) marker and error checking characters(ECC)
  • The receiver responds with an acknowledge if data is good or a no acknowledge if there is an error.
ftp s and baud rate
FTP’s and Baud Rate
  • File Transfer Protocol
    • FTP is used to enable you to exchange files with mainframes, mini’s, or other PC’s. It should be included in the software package that comes with your modem
    • Refer to pages 1046-1047 for variations of FTP.
  • Baud Rate
    • Baud rate is the time that a modem uses as its carrier frequency
    • Baud rate speed is measured in bits per second
      • Example: If baud rate was 2400 and modulated at 2 bits per cycle the bps would be 4800
flow control
Flow control
  • Flow Control (Handshaking)
    • Process by which 2 serial devices verify a conversation.
    • During data transfer there are 2 conversations when flow control is needed, Local(between modem and com port) and end-to-end(between modems).
    • Flow control can come in hardware and software. Look at pg. 1049 to learn about each.
interface signals
Interface Signals
  • CD – Carrier signal detected
  • RD or RX – Receive Data
  • TD or TDX – Transmit Data
  • DTR – Data Terminal Ready
  • DSR – Data Set Ready
  • RTS – Request To Send
  • CTS – Clear To Send
modem commands
Modem Commands
  • Commands of modems are known as the command set.
  • There is no standard for modem commands, but there is a main set that most companies follow. It is the famous AT command set.
  • AT commands are entered through the keyboard.
  • A list of AT commands is on pg. 1052-1053
modem commands1
Modem Commands
  • AT – checks to see that modem is plugged in
  • ATH – Hangs up
  • ATM1 – turns the speaker on
  • ATM0 – turns the speaker off
  • ATD – dials a number
  • ATZ – resets the modem
talking to serial ports and telephone lines
Talking to serial ports and telephone lines
  • Telephone Lines
    • A phone line is guaranteed to handle speeds only up to 2400 baud
    • To verify you are getting the best quality is to call the phone line.
    • If the company thinks the voltage drop is too much, they will condition the line. Conditioning is adding a little voltage to the line to compensate for the voltage drop.
  • Talking to serial ports
    • Data communication equipment is the device that sends data. Data Terminal Equipment is the device that receives data. The connection between the 2 is called the handshake.
    • Chart on page 1054
uart s
  • Heart of many serial ports.
  • It takes parallel information from data buses and makes it serial information to be sent out to a modem.
  • It’s important to know what type of UART is in your COM ports. You can find out the type by checking the diagnostics tab/ More Info button under the Modems applet in the control panel under windows 9x. In Windows 2000 use the Phone and Modem Options applet.
  • For UART’s check pg. 1056
fax modems
Fax Modems
  • Fax technology was invented in 1842 by Alexander Bain
  • Modems send data in single dots which are converted to text.
  • 4 different classes or groups of fax standards
    • Groups 1 and 2 were analog standards for fax modems operating at 300 baud.
    • Group 3 was standardized for digital facsimile devices to communicate over phone lines. Fax transmission had a max speed of 14400 bps.
    • Within group 3, there are several different classes that explain devices using different speeds and methods of communication.
    • Any good fax modem software can run G3
    • Chart of Fax standards on 1058
modem negotiations
Modem Negotiations
  • Standardized series of responses between two modems so they can determine optimal speed, data compression, error detection, etc.
  • Chart pg. 1059-1060
  • TAPI is a set of dynamic link libraries (DLL) that enables Windows 9x to perform telephone functions.
  • A good TAPI installation will configure the modem type, I/O address and IRQ, local phone number, long distance dialing codes, and etc. to enable it to use the modem.
  • TAPI is also used for answering machines, handle faxing, call forwarding, caller ID, and paging
installation and troubleshooting
Installation and Troubleshooting
  • Modems rarely fail, but problems usually are COM port problems
  • Set up and verify the com ports
    • Make sure you install nonconflicting I/O addresses and interrupts.
    • Check modems in device manager to verify the modem is working properly.
    • Windows 9x and 2000 PnP technology makes installation very easy. To do it manually use the add new hardware wizard.
  • Verify the Modem’s BIOS
    • Modern modems all have on board BIOS
    • A better term for BIOS would be command set
    • The BIOS is stored on the flash ROM of a modem.
    • BIOS can be corrupted easy by simply removing and reinserting the modem
set up the correct command set
Set Up the Correct Command Set
    • All that Windows communication programs need to handle a modem are the latest IFN files for your modem and version of Windows.
    • If Windows doesn’t verify the modem try a different brand.
  • Windows 2000
    • For dial configuration use the phone and modem option and the control panel
    • Windows 2000 will not detect some internal modems so you must do manual installation.
when modems break
When Modems Break
  • Check COM port and IRQ port
    • Program called modem doctor is very good for this
    • Modem doctor verifies COM ports and tests your modem and tells what kind of shape its in.
  • If your software says there is no modem
    • Make sure modem is looking at correct port
    • Check that there is no conflict with other devices
    • Reinstall all software and reconfigure your modem.
  • Modem works sporadically
    • Make sure you have the right modem installed.
    • Try different type.
    • Check phone lines in house.